While Ext4 was originally merged in 2.6.19, it was marked as a development filesystem. It has been a long time coming but as planned, Ext4dev has been renamed to Ext4 in 2.6.28 to indicate its level of maturity and paving the way for production level deployments. Ext4 filesystem developer Ted Tso also endorsed Btrfs as a multi-vendor, next generation filesystem and along with the interest from Andrew Morton, Btrfs is planned to be merged before 2.6.29 is released. It will follow a similar development process to Ext4 and be initially marked as development only.

Oracle, Red Hat, IBM and Novell want something that has even more features than ZFS (when combined with a logical volume manager).
But it will take until 2010 until you will see it in production. Maybe the next RHEL will ship it. That is just a guess though.